1. Field of the Invention
The present invention relates generally to a valve train for an internal combustion engine or the like and more specifically to a variable valve timing arrangement therefor.
2. Description of the Prior Art
In a known arrangement such as shown in FIG. 1 of the present application, it has been proposed to operate a poppet valve, such as an inlet or exhaust valve of an internal combustion engine, via a rocker arm 1 which engages a cam 2 at one end and which is pivotally mounted on top of the stem 3 of the valve 4 at the other end. The upper surface of the rocker arm 1 is contoured and adapted to abut a lever 5. The point of abutment with the lever 5 defines the pivot or fulcrum point of the rocker arm. With this arrangement as the cam 2 rotates the rocker arm 1 is cammed to pivot about the fulcrum point defined by the aformentioned contact and induce the valve 4 to reciprocate. To vary the timing and degree of lift of the valve 4, a second cam 6 is provided and adapted to abut the lever 5. The second cam 6 is selectively rotated by a suitable hydraulic motor or the like (not shown). Thus, if the second cam 6 is rotated in a direction to urge the lever 5 to rotate counter clockwise (viz., downwardly as seen in the drawings) the degree of valve lift and the duration that the valve is open will be increased. Rotation of the cam which allows the lever to pivot in the clockwise direction (as seen in the drawings) reduces the valve lift and the duration for which the valve is open.
However, this arrangement has suffered from the drawbacks that the provision of the cam and lever arrangement above the rocker arm increases the overall height of the engine and as the lever/cam arrangement does not permit ready adjustment of the clearance between the rocker arm and the top of the valve stem, a rather large clearance must be provided to allow for thermal expansion, wear etc. This clearance unavoidably leads to the generation of so called "tappet noise", vibration and also tends to deteriorate the valve timing itself.
For a complete disclosure of the arrangement described above, reference may be made to U.S. Pat. No. 3,413,965 which issued on Dec. 3, 1968 in the name of J. M. Gavasso.